Sand molds are one of two kinds: (1) “green” sand molds are smectite clay/water bonded sand mixtures rammed against a pattern to form a desired contour (a top half or cope and a bottom half or drag are booked together to form a complete mold cavity). The sand is a tough, pliable mixture which will hold its molded shape. Molten metal is poured into the mold cavity where it solidifies to form the resultant casting; and (2) “rigid” molds are sand mixtures which can be molded against a pattern and then hardened into a rigid condition. The method of hardening depends on the kind of binder used. Although smectite clay bonded molds can be hardened by air-drying or baking, usually rigid molds are bonded with organic resins which harden into much stronger and harder shapes. Binders are designed to be hardened by several methods. Some are baked; some are cured or hardened by chemical reaction with a reagent; and some are hardened by flushing with a reactive gas.
Green sand molding is the production of molded metal objects from tempered molding sand and is the most diversified molding process used to cast ferrous as well as non-ferrous metal castings. Green sand molding is favored by foundry personnel because it is economical and permits both quality and quantity production, particularly for smaller castings. Castings as large as three to four tons are made successfully in green sand molds; however, as molds become larger, more time is required for the making and assembling of mold parts, consequently, other types of molding are generally favored for the larger castings. The rapid collapsibility of green sand molds makes them much less resistant to the normal contraction of the castings while metal solidification takes place, thus minimizing problems of stresses and strains. Green sand is defined as a water tempered molding sand mixture with plasticity. A green sand mold used for casting iron and steel usually consists of silica sand, a clay binder, and/or an organic binding agent mulled together with temper water and a coal additive. Other useful foundry sands including chromite, zircon and olivine sands.
Cores are sand shapes which are positioned inside the mold to shape the inside of a casting. If a core were not used, the casting would be solid metal and many castings are not solid, but have inside channels or configurations. Cores are usually rigid shapes employing the same kinds of binders and methods described above for rigid molds.
Much as pavement buckles on a hot day, a sand mold or core can buckle due to expansion during the casting operation. The high temperature expansion buckle of the mold wall causes a defect on the casting surface known as a “buckle” or a “scab”. If a core expands too much, the core will crack or craze and metal will enter the crack to form an irregular fin of metal on the cored surface of the casting which must be removed. Obviously, less thermal expansion in a sand is a great advantage. U.S. Pat. Nos. 2,830,342 and 2,830,913 are directed to the excellent thermal stability of carbon sands. These sands are useful together with the additives and binders disclosed herein.
Relatively inexpensive silica sand grains bound together with a suitable binder are used extensively as a mold and core material for receiving molten metal in the casting of metal parts. Olivine sand is much more expensive than silica sand but, having better thermal stability than silica sand, provides cast metal parts of higher quality, particularly having a more defect-free surface finish, requiring less manpower after casting to provide a consumer-acceptable surface finish. Olivine sand, therefore, has been used extensively as a mold and core surface in casting non-ferrous parts in particular and has replaced silica sand in many of the non-ferrous foundries in the United States. Olivine sand, silica sand and combinations thereof also are useful together with the additives disclosed herein.
Spherical or ovoid grain, carbon or coke particles, known to the trade as petroleum fluid coke, also have been used as foundry sands where silica sands and olivine sands do not have the physical properties entirely satisfactory for casting metals such as aluminum, copper, bronze, brass, iron and other metals and alloys. Such a fluid coke carbon sand presently is being sold by AMCOL International Corporation of Arlington Heights, Ill. under the trademark CAST-RITE® and has been demonstrated to be superior to silica sand and olivine sand for foundry use. Each of these spherical or ovoid grain fluid coke carbon sand also are useful, alone or in combination with other types of foundry sands, together with the foundry sand additives disclosed herein.
Roasted carbon sand as described in U.S. Pat. No. 5,094,289, hereby incorporated by reference, is a low cost carbon sand designed primarily for low melting temperature metals, such as aluminum and magnesium. Roasting at 1300°-1400° F., will remove all of the volatile matter which would otherwise be evolved if raw fluid coke were exposed to aluminum poured at 1400° F. Other roasted carbons sands, having the porosity eliminated, are described in this Assignee's U.S. Pat. No. 5,215,143, hereby incorporated by reference. These roasted carbon sands also are useful, alone or in combination with other types of foundry sands, together with the additives disclosed herein.
All of the above-described foundry sands, and mixtures thereof, are suitable for admixture with the acid-treated clay binders of the present invention to form a foundry sand composition suitable to form a shaped mold for receiving molten metal in the manufacturing of shaped metal parts.
One or more binders mixed with the sand are essential to maintain the sand in a predetermined mold configuration. One of the most commonly employed green sand binders is clay, such as a water-swellable sodium bentonite clay or a low swellable calcium bentonite clay. The amount of the clay binder that is used together with the sand generally depends upon the particular type of sand used in the mixture and the temperature of pouring. Silica sand grains expand upon heating. When the grains are too close, the molding sand moves and expands causing the castings to show defects such as “buckles” (a deformity in the casting resulting from excessive sand expansion), “rat tails” (a rough, irregular depression that appears on the surface of a casting or a minor buckle), and “scabs” (a breaking away of a portion of the molding sand when hot metal enters the mold). To overcome this harmful expansion, more clay is added to the sand mixture since the clay contracts upon heating, thereby compensating for the expansion of the silica sand grains. In green sand molding, the reproducibility of the dimensions obtained on the casting are the result of such factors as shrinkage, changes in dimensions of mold cavity, hardness of mold, stability of molding sand, mechanical alignments of flask and maintaining a fixed temperature.